Electrostatic photography



March 22, 1966 H. E. CLARK ELECTROSTAT I C PHOTOGRAPHY Original FiledMay 1, 1961 HIGH VOLTAGE SOURCE 3 Sheets-Sheet 1 yam SOURCE HIGH VOLTAGESOURCE INVENTOR. HAROLD E. CLARK March 22, 1966 H. E. CLARK 3,241,466

ELECTROSTATIC PHOTOGRAPHY Original Filed May 1, 1961 5 Sheets-Sheet 2-/5 HIGH N -'VOLTAGE SOURCE PbwER SUPPLY I INVENTOR. M HAROLD E. CLARKATTORNEY March 22, 1966 v H. E. CLARK 3,241,466

ELECTROSTATIC PHOTOGRAPHY I Original Filed May 1, 1961 5 Sheets-Sheet 3INVENTOR. HAROLD E. CLARK United States Patent 3,241,466 ELECTROSTATICPHOTOGRAPHY Harold E. Clark, Penfield, N.Y., assignor to XeroxCorporation, a corporation of New York Original application May 1, 1961,Ser. No. 106,657, now Patent No. 3,160,746, dated Dec. 8, 1964. Dividedand this application Jan. 29, 1964, Ser. No. 340,987 8 Claims. (Cl.95--1.7)

This invention relates to xerography and, in particular, to compensationfor a non-uniform exposure of xerographic plates. This application is adivision of applicationSerial No. 106,657, filed May 1, 1961, now PatentNo. 3,160,746. A v

In the field of image reproductions by illumination, extensive variationin exposure results in loss of image definition. A particular area inwhich this problem stands out is in the reproduction of displays fromplan position indicators (PPI) in connection with radar systems. In aPPI display, there is a peak of illumination intensity near the centerof the display with a gradual loss of intensity toward the edges.Likewise all targets, objects or other details in the display appearbrighter toward the center and fainter toward the edges of the display.7 In photographic reproduction of such a predictably non-uniformdisplay, several conventional techniques can be used. For instance, agraded density mask can be interposed graded radially from maximumdensity to zero density at the edges. Also the cathode ray spotbrightness of the PPI scope can be decreased at the center and increasedto its maximum at its edges.

In accordance with the present invention, there is devised novelmethods, means and apparatus unique to xerography for improvingreproduction of characteristically non-uniform displays.

material in the form of pigmented electroscopic particles is depositedon the plate to which it is attracted in accordance with the remanentcharge pattern.

In electrostatically charging a photoconductive insulating plate,various irregularities have been noted, in the performance of the plate,related to the charge level obtained. However, little attention has beenpaid to this since development of an electrostatic latent image dependson potential differences between image and nonimage areas and not on thepotential difference between the latent image and the potentialreference for the apparatus. In the past the approach has been to chargeto a level that will allow good image development in the usualsituation.

In attempting to solve the problem of compensating for predictablenon-uniformities, the above mentioned irregularities were examined moreclosely. Examination showed that there was actually greater platesensitivity when a higher level electrostatic charge was used. That is,a photoconductive insulating surface charged to a higher potentialrequired less exposure to provide a given potential difference betweenimage and non-image areas. Thus, in accordance with the presentinvention, this sensitivity variation with charge amplitude has beenfound useful in compensating for characteristic non-uniform illuminationfrom an object to be recorded. Thus an object of the invention is todefine methods, means and apparatus for compensation of non-uniformexposure in xerography.

An additional object of the invention is a method for non-uniformcharging of a xerographic plate.

It is an additional object to define development apparatus forcontinuous tone development of a non-uniformly charged xerographicplate.

It is a still further object to define apparatus for reproducing imagesfrom a plan position indicator.

Further objects and features of the invention will become apparent whilereading the following descriptions in connection with the drawingswherein:

FIG. 1 is a cross section of a simple electrostatic charging apparatusin accordance with the invention;

FIG. 2 is a top view of the charging apparatus of FIG. 1;

FIG. 3 is a bottom view of one embodiment of a corona discharge devicefor charging a circular area on a xerographic plate;

FIG. 4 is a first alternative cross-sectional view of the chargingdevice illustrated in FIGS. 1 to 7;

FIG. 5 is a diagrammatic view of the second embodiment of a coronadischarge device for use in charging a circular area on a xerographicplate;

FIG. 6-is a diagrammatic view of a third embodiment of a coronadischarge device for use in charging a circular area;

FIG. 7 is a fourth embodiment of a corona discharge device for use incharging a circular area;

FIG. 8 is a second alternative cross-sectional view of the chargingdevice illustrated in FIGS. 5 to 7;

7 FIG. 9 is a fifth embodiment of a corona discharge device for use incharging a circular area;

FIG. 10 is a sixth embodiment of a corona discharge device for charginga circular area;

FIG. 11 is a side elevation of a charging apparatus for non-uniformcharging of a rectangular xerographic plate;

FIG. 12 is a first embodiment of a continuous tonedevelopment electrodefor developing non-uniformly charged xerographic plates;

FIG. 13 is a second embodiment of a continuous tonedevelopment electrodefor developing non-uniformly charged xerographic plates;

FIG. 14 is a cross-sectional view of simple developing mechanismaccording to this invention;

FIG. 15 is a diagrammatic illustration of xerographic apparatus forreproducing images having a circular area;

FIG. 16 is a diagrammatic illustration of a second embodiment ofxerographic apparatus for reproducing images having a circula-r area.

A representative simplified charging apparatus, in accordance with theconcepts of the present invention, is shown in FIGS. 1-4. The functionof this apparatus is to deposit electrostatic charge non-uniformly overa circular area of a member to be charged. In particular, the apparatusof FIGS. 1-4 .will deposit an electrostatic charge varying an accordancewith a linear taper. Thus, a low charge is produced in the center and agradually increasing charge is produced toward the circumference with arelatively high charge at the circumference of the charged area.

Referring to FIG. 1, a drive means 10 such as a motor, acting through abelt-pulley combination 9, rotates corona discharge device 12 on supportaxle 11 over xerographic plate 13. Corona discharge device 12 isenergized for discharge by high voltage source 15, while positionedabove xerographic plate 13 which in turn is positioned and disposed onsupport members 16. As may be observed in FIG. 2, rotation of coronadischarge device 12, by support axle 11, causes corona discharge device12 to sweep over a circular area of xerographic plate 13 with a diameterequal to the length of the discharge device.

One embodiment of the corona discharge device is illustrated in FIGS. 3and 4. Further alternative embodiments of the corona discharge devicewill be described below. FIGS. 3 and 4 show bottom and cross-sectionalviews, respectively, of a corona discharge device known in the art as acorotron. Such a device is described in Vyverberg U.S. 2,836,725.conventionally, a corotron comprises a slender conductor or element 17supported inside a conductive shield or grid 18. The shield or grid 18surrounds and incloses element 17, leaving open a narrow axial slit 19along the side which will be positioned adjacent to a surface to becharged. In accordance with the present invention, the axial slit istapered in a curvilinear fashion so that it is relatively narrow at thecenter and relatively wide at the ends. The shield 18 serves to controland limit the charge current produced by the corotron.

The theory of operation of the charging apparatus of FIG. 1 is bestexplained by first referring to FIG. 2. When corona discharge device 12is rotated over the xerographic plate 13, it may be seen that a largerarea is being charged by portions of device 12 toward its ends thantoward its center. This naturally follows from the fact that the area ofa circle increases as the square of its radius. Likewise it may be seenthat the ends of device ,12 move at a higher velocity relative to thesurface being charged than portions of device 12 nearer to the center.To compensate for the relative velocity variation, it is necessary toincrease the electrostatic charging current with an increase in distancefrom the center of the area being charged.

In accordance with the present invention, it is also desired that theresulting charge level on a surface being charged will increase in alinear fashion from the center to the cincnmference. However,considering the necessity of compensating for the relative velocityvariation, as

discussed above, as well as the desired variation in resulting chargelevel, the charging current along the discharge device must vary in acurvilinear fashion. Thus, the charging current must increase at agreater than linear rate toward the extremities of the charging deviceof these figures to deposit a charge having a linear variation.

One of the characteristics of the corotron is that the charging currentthat will flow to a plate or other electrode is dependent upon andvaries directly with the width of the axial slit in the shield. Thus,the corotron of FIG. 3 has a tapered slit with a minimum width at thecenter for a low-charging'current and a maximum width at the ends for arelatively high-charging current. In order that the charging currentincreases in accordance with a taper, the curvilinear taper of the axialslit in the corotron is also curvilinear as illustrated.

In a preferred embodiment, the apparatus of FIG. 1 is designed to chargea xerographic plate for exposure to an image from a plan positionindicator presentation. In this preferred embodiment, the chargingapparatus is designed so that the area to be charged is charged to apotential of about 100 volts from the center out to a one-inch radius.The charge to be effected in this preferred embodiment increaseslinearly along'the radius to 1,000 volts at the circumference of thearea to be charged and may for example extend over area equivalent tothe usual scope face such as one having a seven inch diaming variationsin charging current from a corona discharge device to a surface to becharged. Among these voltage applied to control shields or grids nearthe corona element, the electrical characteristics of the member beingcharged, the field between the corona discharge device and the surfaceto be charged and the like. Some of these factors are adaptable forutilization in accordance with the invention concepts as disclosedbelow.

FIG. 5 illustrates an embodiment of a corona discharge device 12 inwhich the desired curvilinear taper of the charging current is attainedby tapering the diameter of the corona element 17. Thus, the coronaemitting element in the device of FIG. 5 is a conductive wire with arelatively large cross section at the center of rotation of the deviceand a relatively small cross section at the extremity of the device. Asdepicted in FIG. 5, charging device 12 has a length equal to the radiusof the surface area to be charged. In operation, device 12 is rotated bya support member 11 positioned over a point central of the surface areato be charged. As is obvious, this shortened configuration is equallyapplicable to the embodi ment discussed in connection with FIG. 3.Likewise, the

device in FIG. 5 can be extended symmetrically as is the devicediscussed in relation to FIG. 3, but as should be apparent formation ofthe tapered discharge wire is simplified if a device as shown isemployed. These same device 12 in which the curvilinear taper ofcharging current is attained by curving the device 12 in a manner asillustrated bringing it relatively closer to the surface to be chargedat its extremity than at its center of rotation. Thus in FIG. 6, platepositioning supports 16 are situated to hold a plate to be charged sothat its surface is closer to charging device 12 at the circumference ofthe area to be charged than at the center of the area to be charged.

FIG. 7 illustrates still another embodiment of corona discharge device12 in which a corona element 17 is a high resistance filament. thecorona element is connected to the high side of the high voltage supply15. The other end of the corona ele-.

element and the charging current is thus decreased. The

voltage dropping resistor 21, connected to the inside end of the coronaelement, maintains the potential of the element at that end at thedesired minimum level. In order to have a curvilinear tapered currentcharacteristic, the high resistance corona element 21 has an increasingresistance characteristic along its length toward the inside end orrotational center.

FIG. 8 shows in cross section a corona discharge device which mayalternatively be employed in the embodiments of FIGS. 5 to 7. Instead ofthe solid shield 18 used on a corotron as illustrated in FIG. 4, thecharging current may be controlled by a grid or series of wires 18 asillus-' trated in FIG.1 8. Corona discharge devices, constructed asillustrated in FIG. 8, are known in the art as scorotrons- Such a deviceis disclosed in greater detail in Walkup' US. 2,777,957. The embodimentsof the corona discharge device 12, disclosed in FIGS. 5 to 7, maycomprise multiple corona elements 17, as in FIG. 8 as well as singleelements. The shields 18, in these embodiments, may also .be comprisedof a series of longitudinal wires as illustrated in FIG. 8.

FIG. 9 illustrates an additional embodiment of a corona discharge devicecomprising an array of vertical needles 2?;

In this embodiment, one end of.

supported on a rotational member. A voltage divider network 22,connected across high voltage supply 15, is connected to each of theneedles at various taps. The taps on the voltage divider network are soselected that voltages applied to the different needles in the needlearray will produce the desired variation in charging current along thearray. As in the scorotron and as disclosed in the Walkup Patent2,777,957, a screen or grid or wires may be positioned in front of theneedle array for control purposes. A further variation of theembodiment, illustrated in FIG. 9, may be used to vary the chargingcurrent. In this further variation the needles 23, in the needle array,are spaced more widely near the rotational center of the device withcloser spacing toward the end.

Various embodiments in corona discharge devices discussed in relation tothe FIGS. 1 through 9 are also useful in charging a circular area in auniform manner. To use these devices for this purpose, the chargingcharacteristics would have to compensate only for the difference inrelative velocity between the charging device and the surface beingcharged. In some of the commercial applications, in which a xerographicplate is charged by moving a corona discharge device over its surfacethe transport mechanism for the corona discharge device is relativelycomplex. The rotational corona discharge device, in accordance with thepresent invention, may be caused to traverse the surface of axerographic plate by a comparatively simple rotational means. By usingthese rotational corona discharge devices, it is thus possible toimprove and simplify xerographic charging apparatus.

In FIG. 10, a fixed corona discharge grid is illustrated.

This corona dis-charge grid 17 operates on the principle of twointerwound corona discharge elements driven by pulsed direct currentwith the pulses on one element displaced 180 in time from the pulses onthe other element. The pulsed direct current outputs are graphicallyillustrated beside the high voltage output terminals to first and secondspiral corona elements 25 and 26 in FIG. 10. The principles of this typeof corona discharge device are more fully disclosed in Ebert US.2,932,742.- For the purposes of the present invention, the coronadischarge device illustrated in FIG. 10 comprises tWo conductive coronaelements interwound in a spiral. In charging apparatus, in accordancewith the present invention, the corona element windings will beseparated to a greater extent toward the center of the spiral and willbe more closely spaced approaching the circumference. It is noted thatin this configuration, the variation in spacing Will be in accordancewith a linear taper so that the resulted charge variation on the surfacebeing charged, will vary in a linear manner from the center to thecircumference. Since there is no movement between the surface to becharged and the corona discharge elements during charging, relativevelocities need not be considered.

Various combinations of the different features of the corona dischargedevices discussed in relation to FIGS. 1 through 10 are possible. Forexample the corotron of FIG. 3, wit-h its varied slit width, can also bepositioned in an angular relationship to the surface being charged asdiscussed in reference to FIG. 6. Likewise the corotron, with a taperedcorona element illustrated in FIG. 5, could use a high resistancetapered corona element and utilize voltage drop across the element asdiscussed in relation to FIG. 7. These and other combinations are withinthe scope of this invention.

The apparatus illustrated in FIG. 11 is another embodiment of chargingapparatus for non-uniformly charging a xerographic plate along itssurface. This apparatus comprises a corona discharge device 12 andtransport means 27 comprising a reversible worm gear and support members16 for supporting a member to be charged 13,

as well as the transport means 27. In this configuration,

the transport means 27 is supported so that it will bear an angularrelationship to a member being charged as it moves relative thereto. Asthe charging device 12 is 6 transported across the surface of a memberto be charged, it is brought gradually closer to or farther away fromsaid surface. Thus, one end of the surface being charged will receive ahigher charging current than the opposite end. This apparatus is usefulwhen it is desired to charge a fast charge decay photoconductiveinsulator by relative movement of the corona discharge means and then toexpose the plate to an overall illuminating pattern without relativemovement. Since the portion of the photoconductive insulating surfacefirst charged will have decayed to a greater extent by the time ofexposure than that portion of said surface last charged, it isadvantageous to place a greater charge on that portion of the surfacefirst charged. Thus a uniform charge will exist at the time of exposure.

In a xerographic plate non-uniformly charged, in accordance with themethods and apparatus described above, and exposed to a non-uniformillumination pattern, such a presentation from a PPI scope, conventionaldevelopment and printing techniques can be used to produce good linecopy. However, it is sometimes desirable to produce continuous tonereproductions. Continuous tone reproductions may be obtained with theuse of a development electrode such as disclosed in Walkup US.2,573,881. In operation, a development electrode should carry apotential approximately equivalent to the potential in the backgroundareas at the time of development. This prevents development in suchareas. Since in the present invention the surface is sensitized with alinear variation thereacross and exposure produces a, uniform potentialdifference for development purposes across all areas, the backgroundareas will not all have a uniform potential but will vary along asubstantially linear amount following the trend of the variation ininitial charging. It thus becomes necessary to use a developmentelectrode with a potential gradient over its surface that will vary inthe manner of the background potential on the xerographic plate at thetime of development.

FIG. 12 shows a development electrode 29, which has a potential gradientover its surface varying from a minimum level at the center to a maximumlevel at the circumference. This development electrode 29 is comprisedof a graphite disk 30 having a circular area comparable to the area tobe developed. A power supply 31 is connected to a conductive rim 32around the circumference of the graphite disk. The other side of powersupply 31 is connected to reference potential and also through a voltagedropping device 33 to a conductive button 35 located at the center ofthe graphite disk. Current from the power supply will flow from theconductive button 35 through the graphite disk to the conductive rim 32.Voltage drop due to the resistive characteristics of the graphite diskwill cause the potential appearing across the surface of the disk tovary. The voltage dropping device 33 will maintain the center of thegraphite disk at a desired minimum potential. The potential gradientappearing across the surface of the graphite disk can be varied bycontrolling the cross-sectional thickness of the graphite disk 30 alongits radius. The cross-sectional thickness along the radius of the diskmust be such that the potential gradient across the surface of the diskwill present the same potential as the background areas in anelectrostatic latent image to be developed.

FIG. 13 illustrates a second embodiment of a development electrode 29 inaccordance with the present invention. The embodiment of FIG. 13comprises a series of concentric conductive rings 36 connected through avoltage dividing circuit 37 to a power supply 31. Each of the conductiverings is connected to a different tap of the voltage dividing circuit sothat the outermost ring is connected to the highest voltage tap and theinnermost ring to the lowest voltage tap. The voltage gradient acrossthe concentric rings will then be similar to the voltage gradientobtained in the embodiment illustrated in FIG. 12.

In addition to non-uniform charging of a xerographic plate, compensationfor non-uniform exposure can be obtained during development. The methodsand means for exposure compensation in development are similar to andinvolve the same concepts as the methods and means described above inconnection with non-uniform charging.

'Electroscopic particles may be brought to a surface bearing anelectrostatic latent image by a sheet uniformly coated with saidparticles as disclosed by Mayo in U.S. Patent 2,895,847. Such a particledonor sheet '61 is illustrated in FIG. 14. Electroscopic particles 62are coated on donor sheet 61 in a non-uniform manner with the greaterdensity toward the circumference of the surface. A preferred procedurefor forming this non-uniform particle density is to charge the donorsheet 61 with one of the corona discharge devices illustrated anddiscussed in relation to FIGS. 1 to 10. Sheet '61 being made of aninsulating material such as plastic will receive a nonuniformelectrostatic charge. Powder cloud dusting of the sheet 61 withelectroscopic particles 62 will then result in a non-uniformdistribution as illustrated. Preferably loading of the donor iscontrolled so that during development substantially complete particletransfer takes place at edges and a less dense transfer occurs at thecenter.

A further method of non-uniform development involves the use of adevelopment electrode such as illustrated in FIG. 13. Where axerographic plate has been originally charged uniformly, a developmentelectrode will generally carry a uniform potential approximating thelatent image background potential. Variation from this theoreticallyoptimum development potential will vary development density of the imageconsiderably before causing objectionable background development. In apreferred embodiment, a non-uniform electrostatic latent image may beplaced under development electrode rings 36 while a non-uniformpotential is applied thereto. A powder cloud of electroscopic particlesdispersed through the electrode rings 36 will then deposit with greaterdensity in those areas where there is a greater potential differencebetween the rings 36 and the latent image.

- Xerographic apparatus, in accordance with the present invention, isillustrated in FIG. 15. This apparatus generally comprises a moving beltphotoconductive insulating member 38 whichwill stop in the charging andin the exposing positions and will be in motion passing the developing,transfer and erasing positions. With the belt stationary, chargingdevice 39, preferably similar to that disclosed in FIG. 3, is rotatedover a circular area of the belt member 38. The charging device 39imparts a charge over a circular area which has a minimum potentiallevel at the center of the area and a relatively high potential at thecircumference of the area. The belt member 38 is then advanced overtransport rollers 40 to the exposure position. With the belt memberagain stationary, it is exposed to a selective illumination pattern froma PPI 41. After the desired exposure time, the belt 38 is advancedcontinuously past developer feed 42 from which developer particles arecascaded across the electrostatic latent image on the belt member 38. Asthe belt member moves around the following transport roller, excessdeveloping particles are picked up by receptacle 43. After developing,the belt member 38 continues on and is brought adjacent to the printingpaper 45. The printing paper is rolled synchronously against the beltmember, transferring the image to the paper. The printing paper issupplied from a supply reel 46 over sheet rollers 47 and, after imagetransfer, to a fixing station 48 where the image is fixed to the paper.The moving belt 38 then continues on to an erasing device 49 whichremoves residual developing particles and latent image from the beltbefore it returns to the charging position.

Successive copies of PPI presentations, having improved uniformity inimage definition and contrast, are produced by this apparatus.

FIG. 16 illustrates xerograph apparatus 50, for repro ducing a PPIpresentation, comprising a hexagonal xerographic plate drum 51 which isrotated through the various process stations by an intermittent driveapparatus including a drive motor 52 and a solenoid operated clutch 53.The clutch 53, driven by drive motor 52. engages and turns pulley 54.With drive motor 52 tuming continuously, pulley 54 turns and stopsalternately as determined by clutch 53. Pulley 54 is attached to drivebelt which, in turn, drives drum pulley 56. Drum pulley 56 rotates thehexagonal xerographic drum 51 in an intermittent manner as clutch 53engages and disengages. In each step of rotation of drum 51, one of thesix surfaces is charged by a corona discharge device 39 which ispreferably the embodiment disclosed in relation to FIG. 6. A second ofthe six surfaces is exposed to a presentation from a PPI 41. A third ofthe six surfaces is developed by conventional developing apparatus 57. Afourth of the six surfaces is printed by conventional printing means 58.A fifth of the six surfaces is idle and the sixth surface is erased andcleaned of remanent developer and residual image by conventional erasingmeans 59. As should be apparent elements of the various stations aroundthe plate are moved out of the path of the plate when it is indexed to anew station.

This invention is not to be considered as limited to reproduction of PPIpresentations. The methods, means and apparatus are equally applicableto reproduction of other characteristically non-uniform patterns ofillumination.

While the present invention as to its objectives and advantages has beendescribed herein as carried out in specific embodiments thereof, it isnot desired to be limited thereby, but it is intended to 'cover theinvention broadly within the spirit and scope of the appended claims.

What is claimed is: v

1. In a xerographic method for recording a cathode ray plan positionindicator presentation including sensitizing and exposing a xerographicplate, the improvement comprising charging a circular area of aphotoconductive insulating member with "a varying electrostatic chargethat varies inversely with the characteristic illumination intensity ofsaid PPI presentation, and exposing the charged member to saidpresentation so that a latent electrostatic image including compensationfor the variations in said PPI presentation will be formed.

2. A method of compensating for non-uniform exposure of .a xerographicplate comprising charging said xerographic plate before exposure With acorrespondingly non-uniform charge having its highest charge potentialin the areas in which minimum exposure will take place.

3. A development elect-rode for use in continuous tone xerography todevelop a surface charge non-uniformly to compensate for exposure to anon-uniform light image comprising a graphite disk, a conductive rimattached to said disk, a conductive button in the center of said diskand a power supply connected between said rim and said button so thatthe potential of said disk will vary from the rim to the center while inposition during development.

4. A circular development electrode for use in continuous toneXerography comprising a series of concentric conductive rings, a directcurrent source, a resistive voltage divider network connected acrosssaid source and taps connected at successive points on said voltagedivider network .to successive ones of said conductive rings so that thepotential gradient along the radius of said development electrode willvary in accordance with the variation in background potential of anon-uniformly charged xerographic plate to be developed.

5. A method for developing a non-uniform latent electrostatic imagecomprising depositing a non-uniform electrostatic charge on aninsulating member so that the potential level of the charge variesinversely to the nonuniformity of the latent electrostatic image to bedeveloped, applying electroscopic developing particles to saidinsulating member so that said insulating member acquires a greaterdensity of developing material in the areas carrying a higher potentiallevel of charge, and developing the said non-uniform latentelectrostatic image by presenting the said insulating member bearingparticles to said latent electrostatic image so that the electroscopicdeveloper particles will be transferred to the image bearing surface todevelop thereon a substantially uniform image.

6. A development electrode for uniform development of acharacteristically non-uniform electrostatic latent image comprising aseries of concentric conductive rings, a direct current potential source.and a tapped resistive network for applying consecutive potentialincrements to consecutive ones of said conductive rings so that thepotential differential between image areas and said deyelopmentelectrode will be substantially uniform over the entire latent imagebearing surf-ace, said electrode being pervious to developer permittinga powder cloud of electroscopic material to be introduced through theconductive rings to deposit uniformly on the said image areas.

7. Xerog-raphic apparatus for reproducing an image on a substantiallycircular surface portion of a Xerographic plate and particularly adaptedto compensate for predictable non-uniformity in an original to bereproduced comprising a substantially continuous Xerographic platemember, means to charge a circular area on a surface segment of saidplate member with an electrostatic charge that has an increasingpotential characteristic from the center to the circumference of saidcircular area, means to illuminate said plate member with an original tobe reprloduced, means to develop an electrostatic latent image, means totransfer a developed electrostatic image to an image receiving surface,means to erase residual charge and developer from said plate member, andmeans to index said segment of said plate member to each successive oneof said means.

8. Claim 7 in which said substantially continuous xerographic platemember is a substantially hexagonal drum.

No references cited.

EVON C. BLUNK, Primary Examiner.

3. A DEVELOPMENT ELECTRODE FOR USE IN CONTINUOUS TONE XEROGRAPHY TODEVELOP A SURFACE CHARGE NON-UNIFORMLY TO COMPENSATE FOR EXPOSURE TO ANON-UNIFORM LIGHT IMAGE COMPRISING A GRAPHITE DISK, A CONDUCTIVE RIMATTACHED TO SAID DISK, A CONDUCTIVE BUTTON IN THE CENTER OF SAID DISKAND A POWER SUPPLY CONNECTED BETWEEN SAID RIM AND SAID BUTTON SO THATTHE POTENTIAL OF SAID DISK WILL VARY FROM THE RIM TO THE CENTER WHILETHE POSITION DURING DEVELOPMENT.
 7. XEROGRAPHIC APPARATUS FORREPRODUCING AN IMAGE ON A SUBSTANTIALLY CIRCULAR SURFACE PORTION OF AXEROGRAPHIC PLATE AND PARTICULARLY ADAPTED TO COMPENSATE FOR PREDICTABLENON-UNIFORMITY IN AN ORIGINAL TO BE REPRODUCED COMPRISING ASUBSTANTIALLY CONTINUOUS XEROGRAPHIC PLATE MEMBER, MEANS TO CHARGE ACIRCULAR AREA ON A SURFACE SEGMENT OF SAID PLATE MEMBER WITH ANELECTROSTATIC CHARGE THAT HAS AN INCREASING POTENTIAL CHARACTERISTICFROM THE CENTER TO THE CIRCUMFERENCE OF SAID CIRCULAR AREA, MEANS TOILLUMINATE SAID PLATE MEMBER WITH AN ORIGINAL TO BE REPRODUCED, MEANS TODEVELOP AN ELECTROSTATIC LATENT IMAGE, MEANS TO TRANSFER A DEVELOPEDELECTROSTATIC IMAGE TO AN IMAGE RECEIVING SURFACE, MEANS TO ERASERESIDUAL CHARGE AND DEVELOPER FROM SAID PLATE MEMBER, AND MEANS TO INDEXSAID SEGMENT OF SAID PLATE MEMBER TO EACH SUCCESSIVE ONE OF SAID MEANS.